The Space Shuttle Main Engine (SSME) is fueled by liquid hydrogen, which is maintained within the fuel system at a temperature of about −420° F. (−250° C.). This extremely cold internal temperature creates external surface temperatures on the fuel system and hardware (i.e., housings, ducts, joints, etc.) that are sufficiently cold to cause supercooling of the surrounding air, which forms liquid air. This liquid air may run off onto sensitive electronic sensors, or be cryopumped into restricted cavities, both of which can cause numerous operational problems. Therefore, to prevent this liquid air from forming during operation, the exterior surfaces of the SSME fuel system and hardware are typically insulated with liquid air insulation systems.
Current liquid air insulation systems utilize loose-fitting pieces of polyurethane foam assembled around complex-geometry subsystem components that must remain accessible for interim maintenance. These foam pieces are held together by a one-piece sealed cover, called a boot. When maintenance is necessary, the boot can be removed from around the foam pieces, and then the foam pieces can be easily removed. These foam pieces are not made to precisely fit many small details they surround (i.e., bolt heads, plumbing lines, pressure taps, etc.) because to do so would be cost prohibitive, and would complicate the manufacture and installation of the foam pieces considerably. As such, once installed, gaps exist between adjacent foam pieces, and between the foam pieces and the underlying metal components. These gaps are a potential source of, and/or collecting place for, liquid air.
It was initially thought that these gaps would not pose any problems, since any liquid air created therein would be trapped within the boot. However, it has recently been discovered that if any liquid air created by these gaps pools in critical locations, the resultant temperature of the external surface of the liquid air insulation system could fall below the condensation temperature, thereby allowing liquid air or ice to form outside thereof. This would be undesirable, and could create extremely hazardous conditions.
Therefore, it would be desirable to fill the gaps that currently exist between and around the foam pieces in liquid air insulation systems to eliminate these sources of liquid air. It would also be desirable to facilitate easy application of the gap-filling compound as the foam pieces are being assembled so that the gaps can be easily identified and filled. It would be further desirable to have a putty-like gap-filling compound for filling these gaps, so that the compound stays where it is applied.